1. Field of the Invention
The invention relates generally to the watermarking of content that is represented by digital representations and more specifically to the problem of detecting watermarks in digital representations that have been derived via lossy transformations from an original watermarked digital representation.
2. Description of Related Art
Nowadays, the easiest way to work with pictures or sounds is often to make digital representations of them. Once the digital representation is made, anyone with a computer can copy the digital representation without degradation, can manipulate it, and can send it virtually instantaneously to anywhere in the world. The Internet, finally, has made it possible for anyone to distribute any digital representation from anywhere in the world
From the point of view of the owners of the digital representations, there is one problem with all of this: pirates, too, have computers, and they can use them to copy, manipulate, and distribute digital representations as easily as the legitimate owners and users can. If the owners and users of the original digital representations are to be protected against illegal copiers or forgers of the digital representations, the digital representations themselves must be protected from pirates and forgers.
One technique that is widely used to make piracy and forgery more difficult is digital watermarking. A digital watermark is a modification of a digital representation so that it contains additional information. The modification is done in such a fashion that the additional information remains substantially invisible when an analog form of the digital representation is produced by printing or displaying the digital representation, but can be located and read by those who put the additional information into the digital representation. The additional information can be anything the maker of the watermark chooses, but when watermarks are used to make piracy or forgery more difficult, the additional information is typically ownership or copyright information about the digital representation or information that can be used to authenticate the digital representation or an analog form produced from the digital representation. For further information about watermarking, see Jian Zhao, “Look, It's Not There”, in: BYTE Magazine, January, 1997. Detailed discussions of particular techniques for digital watermarking may be found in E. Koch and J. Zhao, “Towards Robust and Hidden Image Copyright Labeling”, in: Proc. Of 1995 IEEE Workshop on Nonlinear Signal and Image Processing, Jun. 20-22, 1995, in U.S. Pat. No. 5,710,834, Rhoads, Method and Apparatus Responsive to a Code Signal Conveyed through a Graphic Image, issued Jan. 20, 1998, and in U.S. Pat. No. 6,359,985, Koch, et al., Technique for marking binary coded data sets, issued Mar. 19, 2002. For an example of a commercial watermarking system that uses the digital watermarking techniques disclosed in the Rhoads patent, see Digimarc Watermarking Guide, Digimarc Corporation, 1999, available at http://www.digimarc.com/support/cswater.htm in August, 2002. For an example of how digital watermarking may be used to authenticate analog forms, see U.S. Pat. No. 6,243,480, Jian Zhao, et al., Digital authentication with analog documents, issued Jun. 5, 2001.
A technical challenge in the implementation of digital watermarks is making them robust. A digital watermark is robust if it still can be detected when the content of the digital watermark has been altered, either by someone with malicious intent, or simply because the content has undergone a lossy transformation, that is, a transformation in which information is lost. Examples of transformations which may be lossy are a transformation from one kind of digital representation of the content to another kind of digital representation or compression of the digital representation of the content. The production of an analog form from the digital representation is always lossy, and physical wear of the analog form may cause further loss of information. When an analog form is scanned to produce a new digital representation, that transformation, too, is always lossy. Because of the inherent lossiness involved in producing and scanning analog forms and because analog forms are subject to physical wear, the robustness of a watermark is particularly important when it is used in the manner described in U.S. Pat. No. 6,243,480 to authenticate an analog document. It is an object of the present invention to improve the robustness of digital watermarks by improving the manner in which they are made and detected.